1. Field of the Invention
The present invention relates to a multicarrier direct sequence code division multiple access (DS/CDMA) system using a turbo code with nonuniform repetition coding. In particular, it relates to a multicarrier DS/CDMA system using a turbo code with unequal diversity order in its code symbols instead of using a convolutional code.
2. Description of the Related Art
With the recent development in mobile communication technology, the analog communication technologies that have been used in prior art, such as time division multiple access (TDMA) and frequency division multiple access (FDMA), are being replaced by code division multiple access (CDMA), a digital communication technology.
CDMA is a method in which a code is assigned to digitized voice information with a broadened frequency band so that each user can transmit signals according to the code. With this method, a voice signal is first divided into bit units to be encoded, and thereafter inserted into communication frequency band. As a result, the accommodation capacity of each channel increases, and the number of users possibly assigned to the same frequency band is increased by 5–10 times compared with the conventional analog-type multiple access technologies. And thus, it is mainly used for mobile communication technology.
CDMA method is employed in IMT-2000 standard, which is being considered to be a standard for next-generation mobile communication such as voice/moving picture data transmission and/or Internet search. However, it has been pointed out that, in CDMA method, the communication quality could be degraded due to the excessive interference caused by different electric powers assigned to multiple users.
A multicarrier DS/CDMA system using a convolutional code has been proposed to prevent the above-described quality degradation in establishing a wideband CDMA system (Y. H. Kim, J. M. Lee, I. H. Song, H. G. Kim and S. C. Kim, “An orthogonal multicarrier DS/CDMA system using a convolutional code,” J. of Korean Electro. Soc., vol. 37TC, no. 4, pp. 295–303, April 2000; D. N. Rowitch and L. B. Milstein, “Convolutionally coded multicarrier DS CDMA systems in a multipath fading channel-part II: narrowband interference suppression,” IEEE Trans. Commun., vol. 47, no. 11, pp. 1729–1736, November 1999). In this system, the available bandwidth is divided into a set of disjoint equiwidth subchannels, and narrowband DS/CDMA waveforms are transmitted in parallel over the subchannels.
The bit error rate of an orthogonal multicarrier DS/CDMA system using a convolutional code described above is relatively improved compared with that of a single-carrier DS/CDMA system in the case that a partial band-interference is being occurred.
In addition to the above-described multicarrier DS/CDMA system using a convolutional code, a multicarrier DS/CDMA system using a turbo code has been introduced to be a more advanced system (A. H. S. Mohammadi and A. K. Khandani, “Unequal error protection on turbo-encoder output bits,” Electron. Letters, vol. 33, no. 4, pp. 273–274, February 1997; Y. M. Choi and P. J. lee, “Analysis of turbo codes with asymmetric modulation,” Electron. Letters, vol. 35, no. 1, pp. 35–36, January 1999). In this system, different energies are assigned to the two kinds of outputs, information symbols and parity check symbols, of a turbo encoder in the additive white Gaussian noise (AWGN) channel to increase the Euclidean distance, and thus improve the communication quality
That is to say, by using a turbo code instead of a convolutional code, the bit error rate (BER; It indicates the system performance, i.e. communication quality.) is improved such that Eb/No becomes to be 0.7 dB at BER of 10−5 and rate of ½. Consequently, it provides a communication quality almost approaching the Shannon limit, and thus, a turbo code is generally used for error correction.
However, the prior multicarrier DS/CDMA system using a turbo code has a problem that a stronger signal energy given to one user causes a stronger interference occurring to the other users, and the communication quality is degraded thereby.
Besides, in case of inducing more energy to increase the Euclidean distance, the fading effect due to the fading channel cannot be reduced very much.